This application claims the priority of German patent document 197 14 364.4-52, the disclosure of which is expressly incorporated herein.
The invention relates to a process for detecting nitrogen oxide (NO) in fluid media.
Several methods are known for removing nitrogen from diesel engine exhaust gases. A catalytic removal of nitrogen can be carried out by using ammonia or hydrocarbons as reducing agents. Adsorber catalysts can remove nitrogen by the adsorption of NO.sub.x, but must be regenerated at regular short intervals by using reducing agents. Also, nitrogen oxide emission can be reduced by returning exhaust gas into the combustion chamber. This effect can be enhanced by enriching the returned exhaust gas with NO.sub.x by using suitable adsorber materials.
But, material-based nitrogen removal processes are based on knowing load-dependent and rotational-speed-dependent momentary NO.sub.x -emissions for metering of the reducing agent and the adsorption/regeneration periods. This can take place either by using an NO-sensor or by using characteristic diagram values filed in a memory of a computer.
Characteristic NO.sub.x -diagrams do not apply to individual engines but only to series of engines so that fluctuations in the crude NO.sub.x -gas content caused by manufacturing may occur while characteristic diagram points are the same. In addition, momentary catalyst condition (temperature, NO.sub.x -load and reducing agent load) may be different while characteristic diagram points are the same. An NO-sensor-controlling regulation would therefore be preferred. When an NO-sensor is used, exhaust gas recirculation rate can be controlled such that minimal NO.sub.x -emission occurs at each engine operating point.
The literature describes a number of sensors for measurement of NO-content in gases. However, only a few are suitable for conditions in real exhaust gas that is hot.
In principle, ceramic solid electrolytes offer a high potential for the application because they are highly selective, resistant to high temperatures and are not expensive to build. This can be seen by the considerable success of the .lambda.-sensor, an oxygen ion conductor based on zirconia. For this principle to be effective for NO, a solid NO.sup.+ electrolyte is required.
U.S. Pat. No. 5,466,350 describes an amperometric thin-film solid-electrolyte detector for NO which is based on the passage of nitrosonium cations (NO.sup.+) through the solid electrolyte. Four electrodes are required in a bipotentiostat arrangement: a first and a second working electrode, a joint reference electrode and a joint counter electrode. In addition, a diffusion barrier is used in front of the working electrode acting as the anode; the diffusion barrier provides that the sensor operates under diffusion-controlled conditions. Only in this manner can it be ensured that the output signal is proportional to the NO-concentration in the gas.
In U.S. Pat. No. 5,466,350, a NO-.beta.-Al.sub.2 O.sub.3 is used as the solid electrolyte and is manufactured in two steps from Na-.beta.-Al.sub.2 O.sub.3. First, the Na-.beta.-Al.sub.2 O.sub.3 is exchanged in an AgNO.sub.3 -melt with Ag.sup.+ to Ag-.beta.-Al.sub.2 O.sub.3. In a second step, Ag-.beta.-Al.sub.2 O.sub.3 is exchanged with NO.sup.+ while utilizing NOCl-ions so that finally NO-.beta.-Al.sub.2 O.sub.3 is obtained. This second step requires a medium which simultaneously has a good solubility for NO.sup.+ as well as for Ag.sup.+ and good oxidation resistance at temperatures about 200.degree. C.
The state of the art according to U.S. Pat. No. 5,466,350 has the following disadvantages:
The bipotentiostat arrangement uses four electrodes. In contrast to a two-electrode arrangement, this represents a clearly more complicated arrangement with respect to design, manufacturing and operation. PA1 So that the ionic current in the solid electrolyte in an amperometric measuring principle is proportional to the NO-concentration in the gas phase, a diffusion barrier must be integrated. PA1 Furthermore, as the result of the principle, the suggested amperometric sensor principle with the diffusion limitation is slow because it is diffusion-dependent. Also, sensor signal quality depends on sensor surface quality (coating by soot particles). In the real exhaust gas of a motor vehicle, a constant and continuously accurate relation between NO concentration and sensor reading can therefore not be achieved. PA1 In addition, a basic disadvantage of the amperometric measuring principle is cross-sensitivity to water vapor which exists in exhaust gas in high concentrations. Because a voltage of at least 1.6 volts--the literature value of 1.31 V relates to the calomel electrode--is required for the NO-oxidation, reactions with water vapor also may take place on the first working electrode (anode). For example: EQU 2H.sub.2 O{character pullout}O.sub.2 +4H.sup.+ +4e.sup.- (1.229 V) EQU 2NO+H.sub.2 O{character pullout}N.sub.2 O.sub.4 +4H.sup.+ +4e.sup.- (1.035 V) PA1 The suggested manufacturing process for the NO.sup.+ -conducting solid electrolyte requires working in aggressive, non-hydrolysis-resistant salt melts and is carried out in a very cumbersome manner in several steps. In particular, cleaning of the solid electrolyte after the ion exchange with water and alcohol required when a salt melt is used is problematic and may damage the electrolyte (hydrolysis of NO to HNO.sub.3). The limitation to 190.degree. C. (starting decomposition of the NO salt melt) and the concentration difference between the melt and the electrolyte as the only driving force makes the complete ion exchange impossible in dense layers of .beta."-Al.sub.2 O.sub.3. However, gas-tight thin layers of pure .beta..DELTA.-Al.sub.2 O.sub.3 are necessary for a technical implementation of the results for a fast sensor. PA1 A basic disadvantage of the exchange reactions in the melt or in the solution is that the moving force of the reaction is represented by the formation of insoluble products, such as AgCl when an Ag.sup.+ -ion conductor is used in an NOCl-containing melt. From preliminary investigations, it is known (R. H. Radzilowski, J. T. Kummer, Inorg. Chem., 8 (1996) 2531; S. Maraun, Diploma thesis, University GH Essen, 1996) that these insoluble residues may precipitate on the solid electrolyte and prevent exchange by passivation. This requires that starting materials are used which are as fine-grained as possible and which, after the exchange, must first be chemically processed and then compacted to molded bodies. As will be illustrated in the following, in the process according to the invention, molded bodies of the desired shape can be used directly. PA1 U.S. Pat. No. 5,466,350 is based on Na-.beta.-Al.sub.2 O.sub.3. In contrast to Li-stabilized Na-.beta."-Al.sub.2 O.sub.3, Na-.beta.-Al.sub.2 O.sub.3 has a different layer sequence and a resulting lower ion conductivity. In addition, it may be affected by hydrolysis. As a rule, it contains high (&gt;1%) parts of NaAlO.sub.2 which is responsible for this water delicacy. A quantitatively complete exchange of Na.sup.+ for Ag.sup.+ is therefore not possible. The remaining Na results in a strong decomposition due to hydrolysis in real exhaust gas and therefore in a fast destruction of the electrolyte in real exhaust gas. PA1 no purely potentiometric measuring principle (.lambda.-sensor); this leads to a slow sensor kinetic and requires an expensive sensor technology and also expensive sensor electronics. PA1 low stability of the solid electrolyte to hydrolysis results in a destruction of the electrolyte during an operation in real exhaust gas; PA1 expensive production process; no complete conversion to an NO-ion conductor is possible. PA1 positioning of a nitrosonium (NO.sup.+)-conducting solid electrolyte between two porous electrodes; PA1 acting upon one of the two electrodes with the NO-containing fluid medium, without insertion of a diffusion barrier; PA1 measuring the voltage difference between the two electrodes as an indication of the NO-concentration in the NO-containing fluid medium. An apparatus corresponding to the above process is also provided.
The Protons formed according to these reactions are also transported through the electrolyte and, like NO, are reduced on the second working electrode, which acts as the cathode. The measured current is therefore composed of the sum of NO.sup.+ and H.sup.+.
With respect to the manufacture of the NO.sup.+ -conducting solid electrolyte according to U.S. Pat. No. 5,466,350, there are the following disadvantages:
Summarizing, the following disadvantages exist for implementing a sensor according to U.S. Pat. No. 5,466,359, particularly with a view to motor vehicle applications:
It is therefore an object of the invention to provide a process for NO-detection in fluid media by means of which the above-mentioned disadvantages of U.S. Pat. No. 5,466,350 are overcome.
This object is achieved by means of a process for selectively detecting NO in fluid media comprising positioning a nitrosonium (NO.sup.+) conducting solid electrolyte between two porous electrodes, acting upon one of the two electrodes with the fluid containing media, and then measuring the voltage difference between the two electrodes. An apparatus is also provided.
The object according to the invention comprises the following process steps:
The process according to the invention overcomes the disadvantages of the state of the art in that a potentiometric and not an amperometric measuring is carried out for NO-detection. As a result, only two electrodes are required. In principle, potentiometric sensors achieve a fast signal reaction because no diffusion equilibrium need be adjusted. Also, the measuring signal is independent of changes of the sensor surface which typically occur during operation in real exhaust gas.
Furthermore, instead of being obtained by way of an ion exchange in the salt melt, the NO.sup.+ -containing solid electrolyte can be obtained directly from a gas phase. This can take place in simple gas-tight reactors and is therefore easy to carry out even in large-scale applications.
The process according to the invention can be used particularly in the exhaust gas of lean-operated as well as .lambda.=1-operated motor vehicles for determining momentary nitrogen oxide concentration.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.